(HealthDay News) — Marijuana use during pregnancy may increase the risk of premature delivery, according to a study published in the July issue of Reproductive Toxicology.

Claire Roberts, Ph.D., a professor at the University of Adelaide School of Pediatrics and Reproductive Health in Australia, and colleagues evaluated data from 5,588 pregnant women in Australia, New Zealand, Ireland, and the United Kingdom. About 5.6 percent of those women reported smoking marijuana before or during pregnancy.

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The researchers found that mothers who still smoked marijuana 20 weeks into pregnancy were more than five times likelier to suffer a spontaneous preterm birth, after adjusting for age, cigarette smoking, alcohol, and their social and economic status (adjusted odds ratio, 5.44). The more often an expectant mother used marijuana, the earlier her baby was born, the researchers also found.

“Not only did continued use of marijuana increase risk for preterm birth, but it also made these births five weeks earlier, on average, with a greater number of women delivering very preterm,” Roberts told HealthDay. “That is much more dangerous for the baby, who inevitably would require admission to a neonatal intensive care unit. Earlier delivery would be expected to increase the baby’s risk for dying and having long-term disabilities.”

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